1. Field of Invention
The present invention relates to a method for machining an electrode workpiece and the electrode workpiece so manufactured. In particular, the present invention relates to a method of machine finishing a frusto-conical disc spring by an electro-erosion process, wherein exact tolerances for the spring are attainable.
2. Prior Art
One of the many uses for frusto-conical disc springs, and more particularly for Belleville springs, is as a control mechanism for a gas pressure regulator. The Belleville spring provides a reference force in the gas pressure regulator for precise control of flow rate and airflow pressure such as is needed in a face piece for a breathing apparatus. Breathing apparatus are particularly useful in hostile environments including those typically encountered by firefighters, airplane crews and the like and precise regulation of the breathing air pressure at the face piece necessitates that the Belleville spring be manufactured to exacting tolerances. In order to achieve such preciseness, as well as fast production time, the Belleville spring of the present invention is produced by electro-erosion using an electro-discharge machining (EDM) process.
The electro-discharge machining process involves providing a high frequency pulsed current across a formed conductive electrode and a grounded workpiece submerged in a dielectric fluid. The electrode is rotated while being moved axially along a work centerline, through the dielectric fluid and towards the workpiece, maintaining a constant gap therebetween. A dampening rod is used to depress resonant vibration in the workpiece during the EDM process while the discharge of an electric spark from the electrode at a chosen frequency and current erodes a portion of material from the workpiece. Distortion in the surface finish of the workpiece due to heat generated during the EDM process is reduced by circulating the dielectric fluid through the gap between the electrode and the workpiece, thereby carrying away the generated heat as well as the eroded material. The power supply frequency, current and electrode movement is numerically controlled to regulate the metal removal and thereby provide the proper surface finish.
Prior to subjecting the workpiece to the EDM process to thereby manufacture the Belleville spring of the present invention, the general shape of the spring is provided by subjecting a piece of steel stock to a press-forming process. Such a process is described in U.S. Pat. No. 3,668,917 to Komatsu et al., wherein the distortion problems frequently encountered in using successive and separate steps of press forming and quenching a piece of steel stock to produce the desired shape of a Belleville spring are overcome. This prior art method provides for simultaneously press-forming and quenching the steel stock placed between a pair of cooperating die members upon heating the steel stock to its austenitizing temperature. The die members apply opposing pressure to the steel stock while they are maintained at a working temperature to rapidly conduct heat, thereby quenching the steel stock under forming pressure. The Komatsu et al process does not completely eliminate distortion in the final product and the resulting frusto-conical disc spring is described as being useful as an automotive clutch. This magnitude of tolerance control is unacceptable for use as a control mechanism, such as a gas pressure regulator where extremely close machining tolerances are required.
U.S. Pat. No. 4,039,354 to Schober describes a Belleville spring formed from a steel blank having a deliberately produced carbon gradient through the blank thickness. The level of carbon determines the transformation temperature at which austenite transforms into martensite, the desired final product. Providing a carbon level at the surface of the steel blank that is higher than the internal carbon level is designed to balance the temperature gradient set up through the blank thickness during quenching wherein the transformation temperature at the surface is lower than the transformation temperature range at the core. This enables the spring core to martensite prior to the spring surface which reduces the formation of internal tensile stresses that frequently lead to cracks and similarly undesirable imperfections in the surface finish. Tailoring the carbon gradient through the thickness of the steel blank to the correct formulation is difficult to regulate.
U.S. Pat. No. 4,135,283 to Kohlhage describes a method of hardening and roughening the surface of a Belleville spring by causing a stream of shot-peen to impinge on the spring surface. The surface is then at least partially smoothed by grinding and polishing in a drum, or by an electro-chemical process. This process does not provide a Belleville spring having a surface finish that is acceptable for use as a control mechanism for use as a gas pressure regulator, and the like.
OBJECTS
It is therefore an object of the present invention to provide an improved frusto-conical disc spring having a uniform cross sectional thickness and a smooth surface finish.
It is another object of the present invention to provide a method for manufacturing a frusto-conical disc spring by an electro-discharge machining process wherein the spring has a uniform cross-sectional thickness and a smooth surface finish.
Still another object of the present invention is to provide a method for manufacturing a frusto-conical disc spring that is acceptable for use as a control for a gas pressure regulator.
Finally, another object of the present invention is to provide a method that can be rapidly carried out to produce a frusto-conical disc spring having a high degree of dimension tolerance such that the unit cost of the spring is relatively low.
These and other objects will become increasingly apparent to those of ordinary skill in the art by reference to the following description and to the drawings.